Agreed with much of what was said, if you have a Diamond mart of some type in you area that has loose stones that may be your best bet, then just find the right setting.I found them to be much more reasonable than the big box jewelry stores.
My wife's setting is platinum which is much more durable and easier to keep shiny than my white gold band although the cost is about double.
We also both went with comfort rings (the sides of our rings are rounded not flat) Much easier to get on and off when the inevitable weight gain starts to come into play.
I agree that certified is a must, get the most you can afford in clarity and color as that what makes the diamond sparkle.
Last couple points, this ring at sometime could end up being a family heirloom that gets passed to you son or daughter.
And probably the most important thing is your girlfriend is going to be holding out this ring to all her friends and family for at least a year, so you need to look like you did the right thing.
